1. Field of the Invention
The present invention relates to a method and system for producing activated carbon. More particularly, the present invention relates to a method and system for producing activated carbon, in which cellulose acylate can be utilized as raw material, and the activated carbon with high adsorptivity can be produced.
2. Description Related to the Prior Art
Polymer films have excellent optical transmittance and flexibility and are capable of forming thin and lightweight films. Owing to this, the polymer films are widely used as optical functional films. In particular, a cellulose acylate film formed of cellulose acylate and the like has toughness and low birefringence in addition to those properties. The film is used as a protection film for a polarizing filter incorporated in a panel, namely, a liquid crystal display (LCD) whose market is recently expanding.
In a polarizing filter production process to produce polarizing filters, approximately 30% of the total amount of supplied films is discarded. The discarded films or waste films are rapidly increasing in accordance with rapid market expansion of the polarizing filters. However, reuse and recycle systems have not been developed for the waste films in comparison with a reuse system of wastepaper and a recycle resin forming system of waste plastics. Waste films are burned or disposed of in landfills at additional waste-processing expense.
However, reuse of the waste films is desired from the viewpoint of recent environmental problems and saving of resources. As methods to reuse the waste films, there are a method to selectively isolate cellulose acylate which is the raw material of the films from the waste films, a method in which the waste films are used as raw materials for molding, and the like. However, in the former method, it is difficult to selectively isolate only the cellulose acylate from the waste films, because several additives contained in the waste films by the film production are extremely difficult to remove. In the latter method, the waste films cannot be used as the raw material for molding. This is because the glass transition point of the cellulose acylate is near to its decomposition point. Cellulose acylate characteristically starts decomposition at the same time as melting.
JP-A 11-171524 discloses a method which reuses wastepaper such as old newspaper and the like as a raw material of activated carbon. However, in many cases, carbonized materials generated by carbonization of waste materials do not have high adsorption property due to the above reasons. Accordingly, most of such carbonized materials are used as soil improvement agents, fuels or the like, but not as adsorbents.
Furthermore, JP-A 7-172808 discloses a method which reuses phenol resin-based wastes as raw materials of activated carbon. There is a known process of decomposing cellulose acylate by application of heat.
WO 2008/090938 discloses a method of processing activated carbon with high adsorptivity from organic material containing cellulose acylate as a main component.
However, only powdered activated carbon is available according to the disclosure of WO 2008/090938. A well-known utilization of the powdered activated carbon is to add the powdered activated carbon in liquid together with compounds such as dyes or raw material before chemical reaction, and to stir the liquid as dispersion or the like, for discoloring of the above various compounds and adsorption of impurity. A preferable system in which the powdered activated carbon is used is a batch system. However, there are other suitable uses of the granular activated carbon which are different from the uses of the powdered activated carbon. For example, the granular activated carbon is suitably used in a known adsorption device for adsorbing solvent with steam by use of activated carbon as gaseous solvent evaporated in the course of solution casting of the cellulose acylate film.
This is because the powdered activated carbon is more difficult to handle than granular activated carbon. The powdered activated carbon may flow away or scatter in an accidental manner, for example, during placement in an adsorption device, during use for adsorption, during desorption by use of steam, and the like. The surface area per unit volume is larger according to the smallness in the granule diameter. The state of containing the activated carbon is tight in a chamber in the adsorption device owing to the powdered form. It is impossible to keep a passage space for gaseous substance to be adsorbed, for example, gaseous solvent in a solution casting system. There arise several problems in the use of the granules with a small diameter in comparison with granules obtained from powdered activated carbon by attachment with binder. A problem lies in the insufficient area of contact of the activated carbon with material to be adsorbed. Also, an unwanted increase in the resistance to the passage of the material to be adsorbed is likely to occur.
Examples of binders of attaching the powdered activated carbon in a granular form include PVA, cornstarch, petroleum pitch, and the like. However, the granular activated carbon formed by use of PVA or cornstarch has low resistance to solvent and low resistance to heat, and are unsuitable in an adsorption device in a solution casting system. This is because the granular activated carbon is degraded or broken by contact with solvent to be adsorbed or steam used for desorption. In contrast, the granular activated carbon formed by use of petroleum pitch has high resistance to solvent and high resistance to heat. However, there is a problem in the use of petroleum pitch as harm to the environment in comparison with the use of PVA or cornstarch.